The proper positioning of the electrode is a critical element in the operation of a consumable electrode furnace, for example a vacuum arc furnace. The operation of a vacuum arc furnace depends upon the control of the arc length or the arc gap which is the distance between the bottom of the electrode and the surface of the pool of molten metal at the top of the ingot. The quality of an ingot formed in such a furnace depends upon the maintenance of a consistent arc length which can be neither to short nor too long.
The problem of maintaining the proper arc length has been dealt with in various ways by furnace manufacturers and users. Initially, the voltage gradient across the gap was used but this proved to be inadequate. Subsequently it was discovered that the frequency and/or duration of periodic fluctuations in the arc voltage signal could be used to improve control of arc length throughout the vacuum arc electrode melting procedure. Several patents have elaborated upon the necessity of controlling the electrode position so as to maintain a predetermined arc length throughout the melt procedure, and on the difficulties that are found in attempting to do this. See, for example, U.S. Pat. Nos. 2,942,045; 2,904,718; 3,372,224; 3,187,078 and 3,186,043. Some of these patents, such as U.S. Pat. Nos. 2,942,045 and 3,187,078, disclose the use of the frequency and/or duration of periodic fluctuations in the arc voltage signal, either alone (U.S. Pat. No. 2,942,045) or in combination with the arc voltage signal (U.S. Pat. No. 3,187,078) as an indication of arc length.
The operation of a consumable electrode furnace is a dynamic process. The electrode is being consumed and therefore must be lowered to compensate for its shortened length. The ingot is of course being formed within the crucible and therefore the rate at which its surface rises affects the speed at which the electrode is lowered toward that surface. The electrode drive means is therefore traditionally set to adjust the electrode feed rate to maintain constant arc length using the frequency and/or duration of periodic fluctuations in the arc voltage as a feedback signal indicative of arc length.
A fundamental problem with arc length regulating systems of this type is that the periodic fluctuations in arc voltage, while having a long term average relationship to arc length, also have a short term random component. Accurate determination of arc length requires that measurements be made of the average frequency and/or duration of fluctuations in the arc voltage signal over a sufficiently long period of time for the random fluctuations to average out to zero.
This creates a dilemma in the design of the arc length control system. If the control system is made responsive and relatively fast acting, it also responds to short term random components in the feedback signal, which in turn leads to ranrandom short term fluctuation of the arc length that can have severe deleterious effects upon the ingot quality. If the control system is made less responsive, it cannot generate the relatively rapid changes in electrode drive speed which are required when the melt rate is changed rapidly as often the case at the beginning and the end of a melt. In this latter case the lag in response of the electrode control system leads to excessively long arc lengths when the melt rate is increasing and to excessively short arc lengths when the melt rate is decreasing.
The existence of the above problem is acknowledged in U.S. Pat. No. 2,942,045 at column 8, line 61 through column 9, line 27, and is touched upon in U.S. Pat. No. 3,187,078 at column 3, lines 11 and 12. Both of these patents disclose control systems which take the second of the two design approaches mentioned above; i.e. the arc length control system is made slow acting by incorporating electrical or electromechanical integrators having long time constants in the control loop.
Analagous problems exist in other types of consumable electrode furnaces such as electroslag remelting (ESR) furnaces. Conventionally the electrode drive speed is based upon measurement of current flow through the slag both or the voltage across it. As in the cae of the vacuum arc furnace, such electrical measurements are subject to undesirable short term or random disturbances. For example, the voltge may drop during start up of another piece of machinery connected to the same electrical power distribution network which is supplying electrical power to the ESR furnace. It would be undesirable for the electrode drive to respond to such a disturbance.